Parent/Child objects behaving oddly


Have been able to make this work for a few other variations, but this is not cooperating. You can see at a few joints the objects pivot instead of rotating despite a clear IR property. Also, some P/R objects wander instead of tracking with the IR chain.

Can someone help me debug this IR animation? Is it the P/R stack? Is it the concept? I just started using Bongo 2.0 a week ago so I admit I could be easily complicating this or missing something.

Any help is appreciated.



PS: it might be easier to debug something like this by sharing screens via Skype. if interested, let me know!

concept 2.1 (share).3dm (2.1 MB)

(Marika Almgren) #2

Hi Matthew,

The biggest problem with your model is the fact that you use groups.
“Grouping” in Rhino is really only a selection helper. It doesn’t really combine the objects. Therefore all the objects inside the group will have their own pivot and animate around it’s own pivot and not work as a group.

The best way to solve it is to make the groups into blocks.

  • Marika

(Luc Adriaenssen) #3

Hey Matthew,

An interesting model! It took me a while to examine – I always enjoy a IK-puzzle :-). In attachment you find a revised version.
I wonder why you blanked the object names. It made it quite hard to figure out what is what. So the first thing I did was naming every object. The yellow color of Layer FTV somewhat interfered with Rhino’s Selected-Object-color (it made it hard to see what is selected), so turned the layer’s color into Gold.
Marika already pointed out the danger of working with groups. In your model some groups joined an object that plays a vital role in the IK-chain (the joints) with other more additional (passive) objects, hence the Animation Manager is unable to display them as a group. That way it all became quite messy, confusing and hard to manage, not so much for Bongo but certainly for yourself.

So I put some structure in the model by using only objects in layer P (the elbows) as joints. I placed their pivots at the location they are expected to rotate. This is not really necessary (as long as they define the right axis) but I feel it makes the model more ‘readable’. The ‘beam’ parts are turned into Blocks (as suggested by Marika) although they only play an addition roll – in this case it just makes things more orderly and for Bongo easier to manage.
Another reason for the dysfunction of your model lies in the absence of 2 joints – yours shows 7 joints in the Animation Manager (use the IK-View) and the revised version has 9 (I made Base 2 rotate-enabled around his Z-axis).

It’s interesting to see that, although the IK-structure is sane (as far as I can see), some joints are still torn apart, certainly near the base.

That’s probable because the basic design of the structure (form a mechanical viewpoint) is too rigid to cope with the translocation of the base. I assume this also causes the IK-chain to jump in a weird twist on opening the file – correct Marika? (or is it a bug?)

Complicating (not to use the term ‘muddling’) is typical for tenderfoots. In this respect, a remark: Although your model “looks” pretty orthogonal, some of your horizontals aren’t precisely horizontal, and some verticals not precisely vertical. Hence some of the Pivots aren’t exactly in line with each other, which can influence the reaction of the IK-chain. My hint is to use Rhino’s Grid Snap while modeling (and positioning Pivots). In this type of models it enhances precision. You can alter the Snap spacing in Rhino’s Options (Tools>Options>Document Properties>Grid).

concept 2.1 (revised).3dm (2.5 MB)

Enjoy practicing.


Thanks for taking a look @marika_almgren and @Luc at this particular model. I had to remove much of the context because this model is under NDA, so apologies for my wiping things clean! I think I still have some development to do with the concept because it’s still tearing. Luc, your revised model shows that. My mechanical engineer doesn’t agree, but I cannot get this model to cooperate as drawn. Sigh…

Really appreciate the feedback though!